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SEMCTL(2) FreeBSD System Calls Manual SEMCTL(2)
NAMEsemctl -- control operations on a semaphore set
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
intsemctl(intsemid, intsemnum, intcmd, ...);
DESCRIPTION
The semctl() system call performs the operation indicated by cmd on the
semaphore set indicated by semid. A fourth argument, a unionsemunarg,
is required for certain values of cmd. For the commands that use the arg
argument, unionsemun is defined as follows:
union semun {
int val; /* value for SETVAL */
struct semid_ds *buf; /* buffer for IPC_STAT & IPC_SET */
u_short *array; /* array for GETALL & SETALL */
};
Commands are performed as follows:
IPC_STAT Fetch the semaphore set's structsemid_ds, storing it in the
memory pointed to by arg.buf.
IPC_SET Changes the sem_perm.uid, sem_perm.gid, and sem_perm.mode
members of the semaphore set's structsemid_ds to match
those of the struct pointed to by arg.buf. The calling
process's effective uid must match either sem_perm.uid or
sem_perm.cuid, or it must have superuser privileges.
IPC_RMID Immediately removes the semaphore set from the system. The
calling process's effective uid must equal the semaphore
set's sem_perm.uid or sem_perm.cuid, or the process must
have superuser privileges.
GETVAL Return the value of semaphore number semnum.
SETVAL Set the value of semaphore number semnum to arg.val. Out-
standing adjust on exit values for this semaphore in any
process are cleared.
GETPID Return the pid of the last process to perform an operation
on semaphore number semnum.
GETNCNT Return the number of processes waiting for semaphore number
semnum's value to become greater than its current value.
GETZCNT Return the number of processes waiting for semaphore number
semnum's value to become 0.
GETALL Fetch the value of all of the semaphores in the set into the
array pointed to by arg.array.
SETALL Set the values of all of the semaphores in the set to the
values in the array pointed to by arg.array. Outstanding
adjust on exit values for all semaphores in this set, in any
process are cleared.
The structsemid_ds is defined as follows:
struct semid_ds {
struct ipc_perm sem_perm; /* operation permission struct */
struct sem *sem_base; /* pointer to first semaphore in set */
u_short sem_nsems; /* number of sems in set */
time_t sem_otime; /* last operation time */
time_t sem_ctime; /* last change time */
/* Times measured in secs since */
/* 00:00:00 GMT, Jan. 1, 1970 */
};
RETURN VALUES
On success, when cmd is one of GETVAL, GETPID, GETNCNT or GETZCNT,
semctl() returns the corresponding value; otherwise, 0 is returned. On
failure, -1 is returned, and errno is set to indicate the error.
ERRORS
The semctl() system call will fail if:
[EINVAL] No semaphore set corresponds to semid.
[EINVAL] The semnum argument is not in the range of valid sema-
phores for given semaphore set.
[EPERM] The calling process's effective uid does not match the
uid of the semaphore set's owner or creator.
[EACCES] Permission denied due to mismatch between operation
and mode of semaphore set.
[ERANGE] SETVAL or SETALL attempted to set a semaphore outside
the allowable range [0 .. SEMVMX].
SEE ALSOsemget(2), semop(2)BUGS
SETALL may update some semaphore elements before returning an error.
FreeBSD 10.1 September 12, 1995 FreeBSD 10.1